In 1970s South Boston, FBI Agent John Connolly (Edgerton) persuades Irish mobster Jimmy Bulger (Depp) to collaborate with the FBI in order to eliminate their common enemy: the Italian mob. The drama tells the story of this unholy alliance, which spiraled out of control, allowing Bulger to evade law enforcement while escalating his power to become the most feared crime lord in Boston and one of the most dangerous gangsters in U.S. history.